The following are known Action Replay Codes for Sonic Adventure on SEGA Dreamcast (Dreamcast). These codes were also known as GameShark Codes and in most cases will work on both devices.

Device Compatibility: Action Replay & GameShark

Infinite Lives
AED34AD4
00000004

Timer always set to 00:00:01
D0C0989A
00000000

Always have 999 gold rings
FE198DC3
000003E7

Turn on Debug mode
77F5C847
00000000